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Thermal-IR Spectral Analysis of Jupiter's Trojan Asteroids

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Thermal-IR Spectral Analysis of Jupiter's Trojan Asteroids 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1238.pdf THERMAL-IR SPECTRAL ANALYSIS OF JUPITER’S TROJAN ASTEROIDS: DETECTING SILICATES. A. C. Martin1, J. P. Emery1, S. S. Lindsay2, 1The University of Tennessee Earth and Planetary Science Department, 1621 Cumberland Avenue, 602 Strong Hall, Knoxville TN, 37996, 2The University of Tennessee, De- partment of Physics, 1408 Circle Drive, Knoxville TN, 37996.. Introduction: Jupiter’s Trojan asteroids (hereafter (e.g., [11],[8]). Had Trojans and JFCs formed in the Trojans) populate Jupiter’s L4 and L5 Lagrange points. same region, Trojans should have fine-grained silicates The L4 and L5 points are dynamically stable over the in primarily amorphous phases. lifetime of the Solar System, and, therefore, Trojans Analysis of TIR spectra by [12] shows that the sur- could have resided in the L4 and L5 regions for nearly faces of three Trojans (624 Hektor, 1172 Aneas, and 911 4.5 Gyr [1]. However, it is still uncertain where the Tro- Agamemnon) have emissivity features similar to fine- jans formed and when they were captured. Asteroid or- grained silicates in comet comae. The TIR wavelength igins provide an effective means of constraining the region is beneficial for silicate mineralogy detection be- events that dynamically shaped the solar system. Tro- cause it contains fundamental Si-O molecular vibrations jans may help in determining the extent of radial mixing (stretching at 9 –12 µm and bending at 14 – 25 µm; that occurred during giant planet migration. [13]). Comets produce optically thin comae that result Trojans are thought to have formed in one of two in strong 10-µm emission features when comprised of locations: (1) in their current position (~5.2 AU), or (2) fine-grained (≤10 to 20 µm) dispersed silicates. Though in the primordial Kuiper belt (KB; ~15 – 30 AU) [2]. Trojans do not have comae, the observed emissivity The surface composition of asteroids can be indicative peaks could arise from a fluffy regolith of fine grain sil- of formation region [3]. Analysis of Trojans in the visi- icates, or silicates suspended in a transparent matrix ble and near-infrared (VNIR; 0.8-4.0 µm) show Trojans [12], [14], [15]. have red or less-red sloped spectra, and likely contain Goals and Hypothesis: The goal of this research is anhydrous silicates [4], [5]. By determining the to constrain the formation region of Trojans by analyz- composition and silicate phase on Trojan surfaces with ing silicate features in the TIR. Had Trojans formed in TIR spectra, we can place constraints on where Trojans the main asteroid belt (near their current location), it is accreted and thus further our understanding of the expected that they would be made of similar material as processes of small body formation. main belt asteroids. Had Trojans and Jupiter Family Background: Trojan surface mineralogy provides a Comets (JFCs) formed in the same region, Trojans means to distinguish between a KB or in situ formation. should have fine-grained silicates in primarily amor- If Trojans formed in or near the Main Belt (MB; 2-5 phous phases, similar to that of JFCs. We hypothesis AU), close to their current position, they are expected to Trojans are more consistent with silicates found on be made of similar material to Main Belt Asteroids JFCs. (MBAs). The largest percentage of outer MBAs are Methods: Mineralogical characteristics aid in con- likely made of hydrated silicates, carbon, and organic straining the origin of Trojans. The Fox and Enx value compounds and some anhydrous silicates (e.g., [6], [7]). has been used in comet research to constrain the struc- Common amongst these asteroid types is either carbo- ture and evolution of circumstellar dust (e.g., [16]). The naceous, metallic, or siliceous material, primarily in trend is more Mg-rich crystalline silicates are found in crystalline or altered form (i.e., aqueous or thermal al- the inner Solar System after condensation, and Fe-rich teration). Had Trojans formed in the outer MB it is ex- silicates to be found in the outer solar system. pected that they are made of similar material that has For this study, we analyze Spitzer Space Telescope been altered in a similar way. Infrared Spectrograph spectra of 11 Trojans: 4709 En- The Nice Model predicts that Trojans are dynami- nomos, 3548 Eurybates, 1437 Diomedes, 588 Achilles, cally linked to KBOs as well as Jupiter Family Comets 617 Patroclus, 659 Nestor, 2797 Teucer, 1998 WD, (JFCs) [2]. Therefore, if Trojans accreted in the primor- 1998 XN77, 4060 Deipylos, and 1867 Deiphobus. dial KB, they would have similar compositions to JFCs. Emissivity peaks associated with olivine and pyrox- Comets that pass near Earth have the advantage of high ene shift from shorter to longer wavelengths with de- resolution spectral measurements as opposed to distant creasing Mg/(Mg+Fe) [17], [18], [19]. The exact loca- Trojans and KBOs. JFCs, such as 9P/Tempel, have a tion of the peak is used to determine the Mg/(Mg+Fe) coma made primarily of CO2, H2O, and sub-micron value for each Trojan in this study (Figure 1). sized silicate grains (e.g., [8]). Typically, silicate-rich We compare Trojan TIR spectra to TIR spectra of comae are dominated by amorphous phases [9], [10], JFC comets such as 9P/Tempel, 29P/Schwassmann- though crystalline silicates have also been detected 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1238.pdf Wachmann, 73P/Schwassmann- Wachmann, 49P/Ar- al. (1993) Mon. Notices Royal Astron. Soc., 264(3), 654- end-Rigaux, and 36P/Whipple. We have used the list of 658. [18] Koike C. et al. (2002) Astron. Astrophys., features identified in [22] to compare Trojan spectral 399(3), 1101-1107. [19] Jager C. et al. (1998) Astron. features to JFC spectral features. These features include Astrophys., 339(3), 904-916. [20] Chihara H. et al. a 15- and 20- µm minimum, a 28 – and 34 – µm maxi- (2002) A&A, 391, 267-273. [21] Koike C. et al. (2003) mum, and the presents and shapes of the 10 µm plateau. A&A, 399, 1101-1107. [22] Kelley M. S. et al. (2017) Results: Olivine emission features in the 10- µm are Icarus, 284, 344-358. consistent with a high iron content for 9 or the 11 Trojan spectra. The remaining three have a low signal to noise, so it could not be determined using this method. The py- roxene mission features in the 10- µm are consistent with a high iron content for up to 5 the 11 Trojan spec- tra, however these five need further analysis. The comparison between Trojans and JFCs shows all Trojans have a 10-µm plateau. However most are round as opposed to the common trapezoid found in JFC spectra. Additionally, most Trojan spectra have 20-µm minima. Unlike JFCs, Trojan spectra tend to not have or have subtle 15-µm minima, 28-µm maxima, and 34-µm maxima. In general JFC spectra tend to be more pro- nounced and sharp. Conclusion: Though Trojan emission features tend to have lower spectral contrast than those of JFCs, the mineralogical results partially suggest an outer solar system origin for Trojans. The sharpness of the features in comet spectra could be due to a higher ratio of crys- talline to amorphous silica as compared to the Trojans or due to observations in two different scattering re- gimes. More work needs to be done on analyzing the spe- cific mineralogy evident in the Trojan spectra. This will be done using a Hapke-Mie radiative transfer code to determine the best fit combination of end-member min- erals. Additionally, lab spectra of end member minerals and of meteorites will be used to compare to Trojan spectra. References: [1] Levison H. F. et al. (1997) Nature, 385, 42-44. [2] Morbidelli A. et al. (2005) Nature, 435(7041), 462-465. [3] Henning Y. (2010) Annu. Rev. Figure 1: (a) TIR spectrum of 9P/Tempel 1 Astron. Astrophys., 48(1), 46. [4] Emery J. P. and with dotted lines indicate crystalline olivine and pyrox- Brown R. H. (2003) Icarus, 164(1), 104-121. [5] Emery ene peaks determined from studies of comet C/1995 O1 J. P. et al. (2011) Astrophys. J, 141(1), 18. [6] DeMeo Hale-Bopp. The variation of peak wavelength position E. E. and Carry B. (2013) Icarus, 226(1), 732-741. [7] with Mg content for crystalline orthopyroxene (b) and Vernazza P. et al. (2017) Astrophys. J, 153(2), 72. [8] olivine (c) peaks observed around 10 µm. The filled Harker D. et al. (2007) Icarus, 190(2), 432-453. [9] symbols mark central wavelengths of strong silicate res- Wooden D. H. (2002) Earth, Moon and Planets, 89, onances and open symbols mark weaker silicate reso- 247-287. [10] Kelley M. S. and Wooden D. H. (2009) nances. Modified from [10], [20], [21]. Planet. Space Sci., 57(10), 1133-1145. [11] Sugita S. et al. (2005) Science, 310, 274-278. [12] Emery J. P. et al. (2006) Icarus, 182(2), 496-512. [13] Salisbury J. W. et al. (1991) Icarus, 92, 280-297. [14] Vernazza P. et al. (2012) Icarus, 221(2), 1162-1172. [15] Yang B. et al. (2013) Icarus, 223(1), 359-366. [16] Hanner M. S. et al. (1994) Astrophys. J, 425(1), 274-285. [17] Koike C. et .
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